Intrinsically safe circuit with low leakage current

ABSTRACT

According to an aspect of this disclosure, an intrinsically safe circuit includes a voltage source, a Zener diode, a transistor, a switching element, one or more resistors, and a current limiting stage. According to this aspect, the intrinsically safe circuit may be configured such that an over-voltage threshold is determined by a voltage across the Zener diode, a base-emitter voltage of the transistor, and a voltage across the one or more resistors.

TECHNICAL FIELD

The present subject matter relates to intrinsically safe electronics,and more particularly, to intrinsically safe barriers.

BACKGROUND

Often times, intrinsically safe barriers are used with loop-poweredfield transmitters, e.g., in the 4 mA to 20 mA range, to limit theenergy provided to such field transmitters when installed in explosiveatmospheres. Explosive atmospheres can be caused by flammable gases,mists, vapors, combustible dusts, or a combination thereof. If acombustible substance mixes with air in sufficient quantities, then anysource of ignition introduced into an environment with an explosiveatmosphere may result in an explosion. Intrinsically safe barriersconnect intrinsically safe equipment suitable for use within theexplosive atmosphere (e.g., intrinsically safe field transmitters,solenoids, proximity sensors, and encapsulated circuitry), to controlsystems for operating, managing, and communicating with theintrinsically safe equipment.

Typically, the control systems, which may be accessible to humans suchas for manipulation of the control software and overall control of thesystem, are not located entirely within the explosive atmosphere.Physically and electrically, intrinsically safe barriers are disposedbetween control systems and the intrinsically safe equipment within theexplosive environment. Typically, intrinsically safe Zener barriersprevent the transfer of unacceptably high energy to the explosiveatmosphere from elements outside of the explosive atmosphere.

Conventional barriers, such as that shown in FIG. 1 , face two typicalchallenges. First, the leakage current flowing through clamping Zenerdiodes may introduce error to a loop current measured by a controlsystem associated with the intrinsically safe barrier. As a result,accuracy of a control loop is decreased. Decreased accuracy of thecontrol loop is generally unacceptable for applications employing anintrinsically safe barrier. Second, in conventional barriers the risk ofdamaging a fuse in the barrier is considerable, e.g., due to overvoltagefrom the power supply, high in-rush currents during power up,hot-plugging, and/or unintentional short circuits during installation ormaintenance.

The description provided in the background section should not be assumedto be prior art merely because it is mentioned in or associated with thebackground section. The background section may include information thatdescribes one or more aspects of the subject technology.

SUMMARY

According to one aspect, an intrinsically safe circuit may comprise aZener diode, a first transistor, a switching element, one or moreresistors, and a current limiting stage. An over-voltage threshold ofthe intrinsically safe circuit may be determined by a voltage across theZener diode, a base-emitter voltage of the transistor, and a voltageacross the one or more resistors.

In some embodiments, the over-voltage threshold may be a sum of thevoltage across the Zener diode, the base-emitter voltage of the firsttransistor, and the voltage across the one or more resistors. Theleakage current across the one or more resistors may be negligible whenan input voltage is below the over-voltage threshold.

In some embodiments, the intrinsically safe circuit may comprise avoltage clamping stage that includes the Zener diode, the firsttransistor, the switching element, and the one or more resistors. Theswitching element may be a MOSFET. The voltage clamping stage mayfurther comprise a pull-down resistor coupled between a gate of theMOSFET and the ground line. A collector of the first transistor may becoupled between the gate of the MOSFET and the pull-down resistor.

In some embodiments, the voltage clamping stage may further include atleast one additional diode. The at least one additional diode, the oneor more resistors, and the Zener diode may be disposed in series betweena positive voltage line and a ground line. The intrinsically safecircuit may further comprise an additional resistor coupled between abase of the first transistor and an intermediate node in the series ofthe at least one additional diode, the one or more resistors, and theZener diode.

In some embodiments, the current limiting stage may comprise second,third, and fourth transistors, a fast-acting transient suppressor, and anegative temperature coefficient thermistor. The fast-acting transientsuppressor and the negative temperature coefficient thermistor may bedisposed in parallel. The fast-acting transient suppressor and thenegative temperature coefficient thermistor can operate to protect thefourth transistor from over-voltage.

According to another aspect, a system may comprise a voltage source, aload, and an intrinsically safe circuit comprising a voltage clampingcircuit and a current limiting circuit. The intrinsically safe circuitmay be configured to accept an input voltage from the voltage source andto maintain a leakage current of less than 10 μA while the input voltageis less than a maximum safe voltage.

In some embodiments, the load may be an intrinsically safe componentwithin an intrinsically safe environment. The intrinsically safecomponent may be a loop-powered field transmitter. The intrinsicallysafe component may be a level magnetostrictive transmitter. Theintrinsically safe circuit may be configured to protect theintrinsically safe component from over-voltage events when the inputvoltage is up to 90 volts.

According to yet another aspect, a method of operating an intrinsicallysafe field transmitter may comprise coupling a voltage supply to anintrinsically safe circuit, coupling an intrinsically safe fieldtransmitter to the intrinsically safe circuit, and powering theintrinsically safe field transmitter by the voltage supply via theintrinsically safe circuit. The intrinsically safe circuit can provide avoltage drop less than 2 volts between an input voltage received fromthe voltage supply and an output voltage delivered to the intrinsicallysafe field transmitter.

In some embodiments, a leakage current of the intrinsically safe circuitcan be less than 10 μA. The intrinsically safe circuit can protects theintrinsically safe field transmitter from over-voltage events when theinput voltage is less than 90 volts.

Other aspects and advantages of the present invention will becomeapparent upon consideration of the following detailed description andthe attached drawings wherein like numerals designate like structuresthroughout the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description below refers to the appended drawings, inwhich:

FIG. 1 is a schematic circuit diagram illustrating a conventionalintrinsically safe barrier for use with loop-powered field transmitters;

FIG. 2 is a schematic circuit diagram illustrating an intrinsically safecircuit with low leakage current and current limiting features forintrinsically safe components, such as loop-powered field transmitters;

FIG. 3 is a schematic circuit diagram illustrating an alternativeembodiment of the intrinsically safe circuit with low leakage currentand current limiting features; and

FIG. 4 is a schematic circuit diagram illustrating another alternativeembodiment of the intrinsically safe circuit with low leakage currentand current limiting features.

In one or more implementations, not all of the depicted components ineach figure may be required, and one or more implementations may includeadditional components not shown in a figure. Variations in thearrangement and type of the components may be made without departingfrom the scope of the subject disclosure. Additional components,different components, or fewer components may be utilized within thescope of the subject matter disclosed.

DETAILED DESCRIPTION OF THE DRAWINGS

The detailed description set forth below is intended as a description ofvarious implementations and is not intended to represent the onlyimplementations in which the subject technology may be practiced. Asthose skilled in the art would realize, the described implementationsmay be modified in various different ways, all without departing fromthe scope of the present disclosure. Still further, modules andprocesses depicted may be combined, in whole or in part, and/or divided,into one or more different parts, as applicable to fit particularimplementations without departing from the scope of the presentdisclosure. Accordingly, the drawings and description are to be regardedas illustrative in nature and not restrictive.

Referring now to FIG. 2 , an intrinsically safe circuit 100 forfacilitating connections (electrical, communications, etc.) between acontrol system and at least one intrinsically safe module/output loaddisposed within a hazardous environment (e.g., an explosive atmosphereor another environment for which intrinsically safe modules aredesired). The intrinsically safe circuit with low leakage current andcurrent limiting features for 4 mA to 20 mA field transmitters(hereafter 4-20 mA field transmitters) is configured to improve thereliability and flexibility of intrinsically safe circuits. Conventionalbarriers, such as that shown in FIG. 1 and discussed above, often facetwo typical challenges: error introduction as a result of leakagecurrent and fuse sensitivity.

The intrinsically safe circuit 100 of the present disclosure providesenergy limitation that is desirable for interfacing with intrinsicallysafe circuitry and further ensures reliable operation of a 4-20 mAcurrent loop (i.e., such as would be suitable for delivery of processcontrol signals). Additionally, a 4-20 mA current loop powered via theintrinsically safe circuit 100 maintains accuracy of the 4-20 mA currentloop and prevents potential damage to barrier components fromover-threshold inrush currents, unintended short circuits, over-voltageoperation, and/or a combination thereof.

Referring again to the schematic circuit diagram of FIG. 1 , the mostcommon configuration of conventional barriers for use in intrinsicallysafe equipment installations is shown. The control system 1 powers acontrol loop. A conventional intrinsically safe barrier 2 is insertedbetween the control system 1 and the 4-20 mA loop-powered fieldtransmitter 3 to limit an amount of energy supplied to the fieldtransmitter 3.

Within the conventional barrier 2, a fuse 4 is used to protect a numberof redundant Zener diodes Z₁ . . . Z_(n). The fuse 4 typically operatesin accordance with IEC (International Electrotechnical Commission)standard 60079-11. The Zener diodes Z₁ . . . Z_(n) operate to clamp thevoltage to a desired value, while a resistor 7 limits the current to thedevice (e.g., a device current 10 in the circuit of FIG. 1 ).

Conventional intrinsically safe barriers incur a leakage current 9 via aparallel path formed by the clamping devices (i.e., the Zener diodes Z₁. . . Z_(n)). This phenomenon results in inaccuracy of the 4-20 mAcurrent read by analog input cards of control modules/equipment of thecontrol system 1. A loop current 8 is measured at the control system 1.The loop current 8 is equal to a sum of the device current 10 throughthe 4-20 mA device (shown in FIG. 1 as a field transmitter) and theleakage current 9 flowing through the Zener diodes Z₁ . . . Z_(n). Theloop current 8 is observed at the control system 1. The leakage current9 increases quickly when a supply voltage approaches a safe outputvoltage U_(o) of the conventional barrier 2.

For example, the safe output voltage U_(o) for conventional barriers,like the conventional barrier 2 of FIG. 1 , is typically 28 V, while themaximum working voltage of examples such as the conventional barrier 2is defined as 24.9 V for a specified leakage current of 10 μA.Similarly, an absolute maximum voltage threshold before the fuse 4 isdamaged is defined as 25.9 V. Even when the 3 V gap between maximumworking voltage and absolute maximum voltage is observed, if the leakagecurrent 9 is 10 μA then about 0.06% error should be expected (seeEquation 1):100×0.01÷16 mA=0.06% error  [Equation 1]

Thus, the amount of error (i.e., 0.06%) introduced during normaloperation at the maximum working voltage is more error than may betolerated while maintaining accuracy for many 4-20 mA loop-powered fieldtransmitters. For example, this amount of error is significantly morethan the 0.01% accuracy desired for level magnetostrictive transmittersthat may be used with an intrinsically safe barrier or circuit.

Additionally, a typical example of the conventional barrier 2 may havean end-to-end resistance of about 300 to 340 ohms. The end-to-endresistance results in a voltage drop of up to 7.2 V (see Equation 2):340 ohms×21 mA=7.2 V  (Equation 2)

If any additional voltage drop occurs across series diodes configuredfor reverse polarity protection, then the voltage drop resulting fromthe conventional barrier 2 exceeds 8 V. A voltage drop of 8 Vsignificantly affects the minimum supply voltage that may be usedconnected by the conventional barrier 2 to the intrinsically safeelectrical environment. Likewise, supply voltage specified by the 4-20mA loop-powered field transmitter 3 is, in turn, limited by the voltagedrop across the conventional barrier 2 supplying power to the 4-20 mAloop-powered field transmitter 3. The 4-20 mA loop-powered fieldtransmitter 3 typically requires a 12.0 V supply voltage for operation.However, given the voltage drop described hereinabove (i.e., 8 V), thepower supply selection for the 4-20 mA loop-powered field transmitter 3is consequently narrowed to 20-24.9 V. At a supply voltage of 24.9 V,the leakage current 9 corresponding to such a supply voltage mayintroduce an amount of error into the loop current 8 that the supplyvoltage may be reduced still further (i.e., below 24.9 V). At this leveland lower supply voltages, the 4-20 mA loop-powered field transmitter 3may not be operational.

Referring again to FIG. 2 , the intrinsically safe circuit 100 isconfigured to facilitate connection between an input/power supply 124(i.e., the control system) and at least one output load 122 (i.e. anintrinsically safe module) while minimizing a voltage drop between apositive input terminal 101 and a positive output terminal 121. Inexemplary embodiments, the maximum voltage drop between the input 124and the output 121 of the intrinsically safe circuit 100 is about 2 V.This maximum voltage drop represents a significant improvement ascompared with the greater than 8 V voltage drop experienced in theconventional barriers, such as the conventional barrier 2 shown in FIG.1 . A load 122, such as a loop-powered field transmitter, anintrinsically safe electrical component, and/or another suitableintrinsically safe load, may be electrically coupled to the output 121.

Further, the intrinsically safe circuit 100 is configured to acceptinput voltages U_(i) up to the maximum safe output voltage U_(o) (i.e.,U_(i)≤U_(o)) while maintaining very low leakage current. An exemplaryembodiment of the intrinsically safe circuit 100 facilitates U_(i)≤U_(o)with leakage current in the tens of nano-amps (nA), such as between 5and 20 nano-amps. In some embodiments, the leakage current may be about10 nA.

The intrinsically safe circuit 100 safely accepts input voltages thatexceed the maximum safe output voltage by around 1 V (i.e.,U_(i)≤(U_(o)+1 V)) while maintaining leakage current of less than 10 μA.Again, this represents an improvement as compared to the ratio of inputvoltage to maximum safe output voltage capable from the conventionalbarrier 2, i.e., U_(i)≤(U_(o)−3 V). Further still, the intrinsicallysafe circuit 100 provides overvoltage protection that interrupts theoutput 121 when the input voltage U_(i) exceeds a value whereat theintrinsically safe circuit 100 cannot guarantee the safe permissibleoutput voltage (i.e., U_(i)>U_(o)+1 V). The intrinsically safe circuit100 tolerates overvoltage up to 90 V or more without risking damage tocomponents. In examples, a threshold for overvoltage protection may bedetermined by selection of the switching element 109 and the electricalproperties thereof. Additionally, the intrinsically safe circuit 100 isconfigured to provide reverse polarity protection.

FIG. 2 is a schematic circuit diagram detailing an example configurationof the intrinsically safe circuit 100 having a voltage clamping stage130 and a current limiting stage 132. When the input voltage U_(i) isapplied through the positive input terminal 101, it reaches the sourceof a switching element 109. In the exemplary embodiment of FIG. 2 , theswitching element 109 is a p-channel MOSFET (metal-oxide-semiconductorfield-effect transistor) that operates as a switching element.Simultaneously, a gate of the MOSFET switching element 109 is pulleddown to ground through pull-down resistor 110. Thereby, the p-channelMOSFET 109 is actuated to an “on” state. A PNP transistor 108 operatesto control the state of the switching element 109.

The input voltage U_(i) is present across a bias circuit 128 of thetransistor 108. In the illustrative embodiment of FIG. 2 , the biascircuit comprises diodes 102, 103, resistors 104, 105, 107, and Zenerdiode 106. In alternative embodiments (see, e.g., FIGS. 3-4 ), thediodes 102, 103 and resistor 104 may be omitted or replaced. In theexemplary embodiment of FIG. 2 , the diode 102, the diode 103, theresistor 104, the resistor 105, and the Zener diode 106 are disposed inseries between a positive input line and a ground line. The resistor 107is connected from a node (between the resistors 104, 105) to a base ofthe transistor 108. A collector of the transistor 108 is connected to anode between the switching element 109 and the pull-down resistor 110. Aresistor 111 is connected from a node (between the resistor 107 and abase of the transistor 108) and the positive input line. An over-voltagethreshold V_(th), which is an input voltage amount required to changethe state of the intrinsically safe circuit 100 to “off”, is equal tothe sum of a voltage across the Zener diode 106 (i.e., the Zener voltageV_(z)) plus a base-emitter voltage V_(be) across the transistor 108 anda voltage drop V₍₅₋₇₎ across the resistors 105, 107 (see Equation 3):V _(th) =V _(z) +V _(be) +V ₍₅₋₇₎  (Equation 3)

However, because a leakage current through the resistors 105, 107 isvery small (i.e., below 1 nA), the voltage drop V₍₅₋₇₎ across theresistors 105, 107 is also below 1 mV and may be omitted in evaluatingproperties of the intrinsically safe circuit 100. As a result, theover-voltage threshold V_(th) may be approximated as equal to the Zenerdiode voltage V_(z) summed with the base-emitter voltage V_(be) (seeEquation 4):V _(th) =V _(z) +V _(be)  (Equation 4)

Under normal operational conditions, the input voltage U_(i) at thepositive input terminal 101 is less than the over-voltage thresholdV_(th)(i.e., U_(i)<U_(th)). Additionally, under normal operationalconditions, the base-emitter voltage V_(be) of the transistor 108 isinsufficient for the transistor 108 to actuate itself to the “on” state.As a result, the gate of the MOSFET switching element 109 remains pulleddown to ground maintaining the MOSFET switching element 109 in the “on”state. However, when the input voltage U_(i) at the positive inputterminal 101 is equal or above the over-voltage threshold V_(th), thebase-emitter voltage V_(be) of the transistor 108 is sufficient for thetransistor 108 to actuate itself to the “on” state. Under thesecircumstances, the gate of the MOSFET switching element 109 is pulled upto the input voltage U_(i), thereby actuating the MOSFET switchingelement 109 to an “off” state.

The intrinsically safe circuit 100 is configured such that the safeoutput voltage U_(o) approximately equals the Zener voltage (i.e.,U_(o)˜=V_(z)). In the exemplary configuration of FIG. 2 , a maximumvoltage at a drain of the MOSFET switching element 109 is theover-voltage threshold V_(th) (i.e., V_(th)=V_(z)+V_(be)). Excessvoltage is compensated for by a diode 112 disposed downstream of theMOSFET switching element 109 (see Equation 5 below). The diode 112 maybe selected to have a forward voltage equal or larger than thebase-emitter voltage V_(be) of the transistor 108 such thatV_(be)=V_(D). Thus, the resulting voltage at the output of the voltageclamping stage 130 is U_(o)=V_(z).U _(o) =V _(th) −V _(D) =V _(z) +V _(be) −V _(D)  (Equation 5)

A leakage current to ground through the voltage clamping stage 130 isabout 40 nA when the input voltage U_(i) equals the Zener voltage V_(z)of the Zener diode 106 (i.e., U_(i)=V_(z)). Again, this represents animprovement as compared to the conventional barrier 2 of FIG. 1 ,because the intrinsically safe circuit 100 safely accepts input voltagesthat exceed the maximum safe output voltage by around 1 V (i.e.,U_(i)≤(U_(o)+1 V)) while maintaining leakage current of less than 10 μA.

The intrinsically safe circuit 100 tolerates relatively higher inputvoltage U_(i), in excess of the expected safe output voltage U_(o). Asmentioned hereinabove, the input voltage U_(i) at the positive inputterminal 101 is less than the over-voltage threshold V_(th), (i.e.U_(i)<U_(th)). Therefore, the base-emitter voltage V_(be) of thetransistor 108 is sufficient for the transistor 108 to actuate itself tothe “on” state. Under these circumstances, the gate of the MOSFETswitching element 109 remains pulled down to ground thereby maintainingthe MOSFET switching element 109 in an “off” state.

Referring still to FIG. 2 , the current limiting stage 132 is configuredsuch that a transistor 114 (or transistor group and/or pair) operates asa current switch, and a transistor 118 operates as a current controlmechanism. Initially, the transistor 114 is actuated to the “on” statedue to a fixed current through a resistor 113 biasing a base of thetransistor 114. Before current flows, a voltage across a sensingresistor 115 is relatively small, and consequently a base-emittervoltage V_(be) is not sufficient to actuate the transistor 118 to an“on” state. As the current increases, due to inrush current from powerup or an excessive load during normal operation, a voltage across thesensing resistor 115 increases until the base-emitter voltage V_(be) ofthe transistor 118 is sufficient to actuate the transistor 118 to an“on” state. When the transistor 118 enters the “on” state, thebase-emitter voltage V_(be) of the transistor 114 is pulled down anamount sufficient to partially actuate the transistor to an “off” state.The ohm-value of the sensing resistor 115 determines a current limit forthe current limiting stage 132, i.e., R₁₅=V_(be)÷I_(limit). Accordingly,the transistors 114, 118 and the resistors 113, 115 manage the limitcurrent I_(limit) that is allowed to flow through the intrinsically safecircuit 100.

Given that the base-emitter voltage quality of transistors typicallychanges with variation in temperature, an NTC (negative temperaturecoefficient) thermistor 116 is utilized to maintain a stable limitcurrent I_(limit). The NTC thermistor 116 is disposed in parallel withthe sensing resistor 115 in order to compensate for the change of thebase-emitter voltage V_(be) with temperature increase or decrease. Theparallel NTC thermistor 116 reduces a tolerance for the current limitingstage 132 thereby improving accuracy of a control current. Whentemperature decreases the V_(be) saturation also increases. Due to theincreased V_(be) saturation, higher current is required to partiallyactuate the transistor 118 to an “on” state, unless the resultingparallel resistance also increases at lower temperature. In other words,the magnitude of V_(be) saturation and parallel resistance (bothresulting from operation/behavior of the NTC thermistor 116) increase ordecrease in correspondence with one another (i.e., the magnitude ofV_(be) saturation and parallel resistance both increase when temperaturedecreases and both decrease when temperature increases.

In exemplary embodiments, one or more dedicated subcircuits may beincluded in the current limiting stage 132 to improve the accuracy oflimit current I_(limit) linearization. A fast-acting transientsuppressor 117 is disposed in parallel with the transistor 118 (and theNTC thermistor 116) to protect the base-emitter junction of thetransistor 118 from overvoltage events.

A fuse 120 ensures that, in a case of failure of the current limitingstage/circuit 132, the safe output current I_(o) is not exceeded.Further, a low value resistor 119 may be included in exemplaryembodiments to meet standards set in IEC 60079-11.

FIG. 3 illustrates an alternative embodiment of the intrinsically safecircuit 100. In the embodiment of FIG. 3 , voltage control diodes 123and 124 are added. The first voltage control diode 123 is disposedwithin the voltage clamping stage 130, in series, between the resistor105 and the Zener diode 106. The second voltage control diode 124 isdisposed within the current limiting stage 132 downstream of the diode112 of the voltage clamping stage 130. The diodes 112 and 124 may bedisposed in either or both of the stages 130, 132.

FIG. 4 illustrates another alternative embodiment of the intrinsicallysafe circuit 100 that includes the voltage control diodes 123 and 124while omitting the diodes 102, 103 and the resistor 105. In the exampleof FIG. 4 , the voltage control diode 123 is disposed between theresistor 104 and the Zener diode 106, as a result of the omission of theresistor 105. In FIGS. 3 and 4 , the addition of the diodes 123, 124increases the V_(th) while maintaining the safe output voltage U_(o) asapproximately equal to the Zener voltage V_(z). Still further, theexemplary embodiments of FIGS. 3 and 4 produce still lower leakagecurrent (e.g., U_(i)=U_(o)). A voltage drop across the diode 124compensates for any increase in the over-voltage threshold V_(th)resulting from the addition of the diode 123. Therefore, the presence ofthe diode 124 effectively balances the voltage drop (between thepositive input terminal 101 and the positive output terminal 121) andleakage current experienced by the intrinsically safe circuit 100.

The embodiment(s) detailed hereinabove may be combined in full or inpart, with any alternative embodiment(s) described.

INDUSTRIAL APPLICABILITY

The above disclosure may represent an improvement in the art byproviding an intrinsically safe circuit, with low leakage current,capable of operating reliably under more variable electrical conditionsthan conventional barriers. The present disclosure contemplates anintrinsically safe circuit that provides superior performance with lowerleakage current than conventional barrier devices, including when thedisclosed intrinsically safe circuit 100 is powered with a higher supplyvoltage than is acceptable for conventional barrier devices.Conventional devices would likely sustain damage if powered by inputvoltages of 26.6 V or greater, while the intrinsically safe circuit 100of the present disclosure is configured to remain undamaged even if upto 90 V of input voltage are applied thereto. However, in part to dealwith overvoltage scenarios, the intrinsically safe circuit 100 isfurther configured to disable the output 121 thereof when the inputvoltage exceeds 28.3 V or another appropriate over-voltage thresholdV_(th). The intrinsically safe circuit 100 is configurable to implementprotections at over-voltage threshold V_(th) of 22 V, 24 V, 27 V, 28 V,30 V or another suitable or desirable voltage level.

The voltage drop of the intrinsically safe circuit 100 describedhereinabove is lower than the conventional barrier(s) 1, such that, incontrast with conventional barriers, the intrinsically safe circuit 100,may operate with field transmitters requiring a 12 V power supply.Conventional devices capable of delivering low output current oftenresult in a larger voltage drop due to the typical series resistanceneeded to limit the current of such devices. These features ofconventional barriers eliminate any choice of barriers for fieldtransmitters requiring a minimum supply voltage of 12 V and an outputcurrent of less than or equal to 50 mA. The intrinsically safe circuit100 of the present disclosure represents an improvement in the art byproviding a circuit suitable for use in an intrinsically safe barrierapplications and capable of supporting field transmitters requiring aminimum supply voltage of 12 V and an output current of less than orequal to 50 mA. Moreover, the present disclosure contemplates adjustmentof the intrinsically safe circuit 100 for compatibility with otherintrinsic safety parameters. Modifying the Zener diode 106, such as byadjusting the Zener voltage via replacement thereof, results in one ormore different safe output voltages U_(o). Similarly,modifying/exchanging the sensing resistor 115 results in one or moredifferent safe output currents J.

While some implementations have been illustrated and described, numerousmodifications come to mind without significantly departing from thespirit of the disclosure, and the scope of protection is only limited bythe scope of the accompanying claims.

To the extent that the terms include, have, or the like is used, suchterms are intended to be inclusive in a manner similar to the termcomprise as comprise is interpreted when employed as a transitional wordin a claim. Relational terms such as first and second and the like maybe used to distinguish one entity or action from another withoutnecessarily requiring or implying any actual such relationship or orderbetween such entities or actions.

Phrases such as an aspect, the aspect, another aspect, some aspects, oneor more aspects, an implementation, the implementation, anotherimplementation, some implementations, one or more implementations, anembodiment, the embodiment, another embodiment, some embodiments, one ormore embodiments, a configuration, the configuration, anotherconfiguration, some configurations, one or more configurations, thesubject technology, the disclosure, the present disclosure, othervariations thereof and alike are for convenience and do not imply that adisclosure relating to such phrase(s) is essential to the subjecttechnology or that such disclosure applies to all configurations of thesubject technology. A disclosure relating to such phrase(s) may apply toall configurations, or one or more configurations. A disclosure relatingto such phrase(s) may provide one or more examples. A phrase such as anaspect or some aspects may refer to one or more aspects and vice versa,and this applies similarly to other foregoing phrases.

The disclosed systems and methods are well adapted to attain the endsand advantages mentioned as well as those that are inherent therein. Theparticular implementations disclosed above are illustrative only, as theteachings of the present disclosure may be modified and practiced indifferent but equivalent manners apparent to those skilled in the arthaving the benefit of the teachings herein. Furthermore, no limitationsare intended to the details of construction or design herein shown,other than as described in the claims below. It is therefore evidentthat the particular illustrative implementations disclosed above may bealtered, combined, or modified and all such variations are consideredwithin the scope of the present disclosure. The systems and methodsillustratively disclosed herein may suitably be practiced in the absenceof any element that is not specifically disclosed herein and/or anyoptional element disclosed herein.

It should be understood that the described instructions, operations, andsystems can generally be integrated together in a singlesoftware/hardware product or packaged into multiple software/hardwareproducts.

The use of the terms “a” and “an” and “the” and “said” and similarreferences in the context of describing the subject matter of thepresent disclosure (especially in the context of the following claims)are to be construed to cover both the singular and the plural, unlessotherwise indicated herein or clearly contradicted by context. Anelement proceeded by “a,” “an,” “the,” or “said” does not, withoutfurther constraints, preclude the existence of additional same elements.Recitation of ranges of values herein are merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g., “such as”) provided herein, isintended merely to better illuminate the disclosure and does not pose alimitation on the scope of the disclosure unless otherwise claimed. Nolanguage in the specification should be construed as indicating anynon-claimed element as essential to the practice of the disclosure.

What is claimed is:
 1. An intrinsically safe circuit comprising: a Zenerdiode, a first transistor, a switching element, one or more resistors,and a current limiting stage, wherein an over-voltage threshold isdetermined by a voltage across the Zener diode, a base-emitter voltageof the transistor, and a voltage across the one or more resistors. 2.The intrinsically safe circuit of claim 1, wherein the over-voltagethreshold is a sum of the voltage across the Zener diode, thebase-emitter voltage of the first transistor, and the voltage across theone or more resistors.
 3. The intrinsically safe circuit of claim 2,wherein a leakage current across the one or more resistors is negligiblewhen an input voltage is below the over-voltage threshold.
 4. Theintrinsically safe circuit of claim 1, wherein the intrinsically safecircuit comprises a voltage clamping stage that includes the Zenerdiode, the first transistor, the switching element, and the one or moreresistors.
 5. The intrinsically safe circuit of claim 4, wherein theswitching element is a MOSFET, and wherein the voltage clamping stagefurther comprises a pull-down resistor coupled between a gate of theMOSFET and the ground line.
 6. The intrinsically safe circuit of claim5, wherein a collector of the first transistor is coupled between thegate of the MOSFET and the pull-down resistor.
 7. The intrinsically safecircuit of claim 6, wherein the voltage clamping stage further includesat least one additional diode.
 8. The intrinsically safe circuit ofclaim 7, wherein the at least one additional diode, the one or moreresistors, and the Zener diode are disposed in series between a positivevoltage line and a ground line.
 9. The intrinsically safe circuit ofclaim 8, further comprising an additional resistor coupled between abase of the first transistor and an intermediate node in the series ofthe at least one additional diode, the one or more resistors, and theZener diode.
 10. The intrinsically safe circuit of claim 1, wherein thecurrent limiting stage comprises second, third, and fourth transistors,a fast-acting transient suppressor, and a negative temperaturecoefficient thermistor.
 11. The intrinsically safe circuit of claim 10,wherein the fast-acting transient suppressor and the negativetemperature coefficient thermistor are disposed in parallel.
 12. Theintrinsically safe circuit of claim 11, wherein the fast-actingtransient suppressor and the negative temperature coefficient thermistoroperate to protect the fourth transistor from over-voltage.
 13. A systemcomprising: a voltage source, a load, and an intrinsically safe circuitcomprising a voltage clamping circuit and a current limiting circuit,wherein the intrinsically safe circuit is configured to accept an inputvoltage from the voltage source and to maintain a leakage current ofless than 10 μA while the input voltage is less than a maximum safevoltage.
 14. The intrinsically safe circuit of claim 13, wherein theload is an intrinsically safe component within an intrinsically safeenvironment.
 15. The intrinsically safe circuit of claim 14, wherein theintrinsically safe component is a loop-powered field transmitter. 16.The intrinsically safe circuit of claim 14, wherein the intrinsicallysafe component is a level magnetostrictive transmitter.
 17. The powercircuit of claim 14, wherein the intrinsically safe circuit isconfigured to protect the intrinsically safe component from over-voltageevents when the input voltage is up to 90 volts.
 18. A method ofoperating an intrinsically safe field transmitter, the methodcomprising: coupling a voltage supply to an intrinsically safe circuit;coupling an intrinsically safe field transmitter to the intrinsicallysafe circuit; and powering the intrinsically safe field transmitter bythe voltage supply via the intrinsically safe circuit, wherein theintrinsically safe circuit has a voltage drop less than 2 volts betweenan input voltage received from the voltage supply and an output voltagedelivered to the intrinsically safe field transmitter.
 19. The method ofclaim 18, wherein a leakage current of the intrinsically safe circuit isless than 10 μA.
 20. The method of claim 18, wherein the intrinsicallysafe circuit protects the intrinsically safe field transmitter fromover-voltage events when the input voltage is less than 90 volts.